MSSL Astrophysics Group Beginner's Guides

A Beginner's Guide to Redshift

See this GIF animation illustrating the expansion of the Universe (62kb).

The Universe is filled with millions of galaxies. In 1912, V. M. Slipher discovered that all other galaxies in the Universe looked `red'.

The Doppler Effect.
You have probably noticed that the pitch of a police car's siren is high as the car approaches, and falls as it moves away. This is known as the `Doppler effect', and the same thing happens with light waves as well as sound waves. If that police car was moving fast enough (a few thousands of kilometers per second or more), you would see that its colour would be more `blue' as it approached you, and `red' as it moved away. Thus it seems that everything beyond our local area of the Universe, is moving away from us.

Hubble's Law.
Later, E. Hubble and M. Humason found that the fainter a galaxy was, the faster it was moving away (ie. the greater its redshift). The brightness that we measure for a galaxy indicates how far away it is (ie faint galaxies lie further away and vice-versa), therefore this relationship suggests that the `recession' velocity of a galaxy (ie the speed at which it moves away) depends on its distance from us. This is illustrated opposite where the arrows represent the positions of galaxies relative to ourselves in two dimensions. The `redness' of the arrows represents the size of the redshift, and the length of the arrows the recession velocity.

An Expanding Universe.
The fact that everything else is rapidly moving away from us does not (necessarily) mean that we are at the centre of a fleeing Universe (was it something we said?). The most likely answer is that we are part of an expanding Universe, where all galaxies are moving away from each other. For a simple analogy, think of the Universe as a bubble with the galaxies scattered around. As the bubble grows, all of the galaxies move away from each other - so which ever galaxy you look out from, the rest of the Universe will always be redshifted. (The colours in this diagram are random to help you follow them from bubble to bubble.) See our animation of the Big Bang and the expanding Universe (62kb) for an illustration of this idea.

Other Extragalactic Astronomy pages: